For reasons of scaling, the use of planar MOSFET structures in device generations with a 65 nm design rule and smaller is becoming more and more complicated due to so-called short channel effects.
An improvement of device characteristics can be obtained by application of a finFET structure. In a finFET, on top of an insulating layer a (relatively narrow) silicon line (a fin) is created between a source region and a drain region to serve as a channel. Next, a line-shaped control gate is created which crosses the fin. The control gate, which is separated from the fin by a thin gate oxide film surrounds (in cross-section) both the sidewalls and the top of the fin, which allows for a relatively large field effect by the gate on the fin channel.
For Flash memory, an application of a finFET structure as described above has been disclosed in U.S. Pat. No. 6,768,158. Between the control gate and the fin material a charge trapping layer stack is positioned. Here, the charge trapping stack layers follow the profile of the fin along both the sidewalls and the top of the fin. The control gate layer follows the profile of the charge trapping stack layers. The charge trapping layer stack comprises either a poly-silicon floating gate or a layer stack of silicon dioxide-silicon nitride-silicon dioxide for trapping charge. Under the control of the control gate, carriers can be transported from the channel to the charge trapping layer to write (or program) data in the Flash memory cell. The mechanism for such a transport of charge depends on the type of charge trapping layer stack: for a poly-silicon floating gate transport takes place by hot carrier injection or by direct tunneling, for a silicon nitride layer by direct tunneling of carriers.
Also, a mechanism for a reversed action to erase the data (charge) from the charge trapping layer is available. Further, a sensing or reading action is available which is used for detecting whether charge is stored in the charge trapping layer. The structure of the non-volatile memory element as disclosed has the disadvantage that data retention, i.e., retention of charge, in the charge trapping layer stack is adversely affected by the above-mentioned method of sensing the charge (reading the stored data). In the finFET of U.S. Pat. No. 6,768,158, sensing the charge is done by the same control gate that is used for storing charge. By comparison with a threshold voltage it can be determined what data is stored (e.g., a bit value 0 or 1). Disadvantageously, this sensing action involves biasing of the control gate, which stimulates stored charge to leak away from the charge trapping layer.